Starter removal and location please?

If it really was because of the rain and/or the cold, run the garden hose over the entire car and see if you can get the fuse to blow and if so perhaps it is because of something getting wet. Or if it is because of the cold, back it out of the garage and leave it for a few hours until it gets cold, and then try it again. That fuse #8 is hot all the time from the battery but the starter relay and the starter motor which includes the starter solenoid is on that circuit, so if and when you can recreate the fuse blowing, see if removing the starter relay to eliminate the starter relay, the starter and the starter solenoid and the wire as the culprit as the reason for the fuse to blow. If at anything, purchase a new starter relay and either go ahead and install it or keep it ready just in case this does occur again or if you do want to test first but just in case that it is a shorted starter relay that is causing the fuse to blow.
If though, the fuse still blows with the starter relay removed you could also eliminate the starter, solenoid and the wire to it as the problem, then you would have to switch your focus to what the ignition switch feeds when the ignition key is only in the start position, such as the transmission control module on one wire from the ignition switch and also a fuse #17 in the junction block which feeds several items, so to eliminate that as the problem or anything after fuse #17 is to remove it while recreating the fuse #8 blowing by turning the key to start when running the garden hose over everything and/or allowing the car is be cold. You may not be able to recreate the fuse #8 blowing. With that done said, you would either have to wait until the problem comes back and know what relays or fuses to remove in order to test or just be well prepared on what to try, which is probably not the best approach but you may be limited to that approach, so they would be the trying a new starter relay. And things along that line.